I have a set of rasters (geologic risk) from which I want to extract zones with a specific color, clipping them with a polygon (administrative borders), outputting a shapefile.
I'm using QGis (and GDAL from command line).
I know I can use clipper, polygonize and show only features with the value I need, but:

it takes ages to polygonize the raster, and I don't want much of the information in it;

clipper doesn't work because the polygon border is larger than each raster, but fits inside the sum of all rasters

to make things complicated, the zone I would like to extract don't have the same color value in all rasters (but do have the same rgb color)

I think I got my answer: gis.stackexchange.com/questions/27359/… I simply have to rasterize my polygon (I addedd a field with value=1), then use raster calculator to do: mask * map = color value then I can use polygonyze (and do a join)
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sanzoghenzoNov 20 '12 at 16:17

it is a different enough question that you should accomplish the outcome and then post as an answer below and check the answer as correct. that is how we roll here at GSE
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Brad NesomNov 20 '12 at 17:50